ABSTRACT A major barrier to effective treatment of the central nervous system (CNS) in most inherited diseases is that pathology is present throughout the brain because the metabolic defect is present in most brain cells. Thus, global distribution of the gene or gene product is required. Several gene therapy strategies are being investigated for treatment of global brain lesions. However, all of the approaches have significant shortcomings, which become apparent in large animal models of human diseases. Development of more effective treatments in these models will facilitate translation into clinical usage. This project will investigate AAV vector mediated gene delivery into the brain by infusion into the cerebrospinal fluid (CSF), which can result in disseminated delivery of a gene in a large animal brain. The disease model to be evaluated is alpha-mannosidosis (AMD) in the cat, a lysosomal storage disorder (LSD) caused by mutations in the lysosomal enzyme (LE) structural gene, alpha-mannosidase (MANB). The strategy for treatment is based on cross-correction, in which transfer of a normal copy of the MANB cDNA into AMD cells results in the metabolic correction of those gene-transduced cells. Furthermore, the genetically corrected cells release normal MANB enzyme, which is taken up by surrounding cells and corrects them metabolically as well. This well-established cross-correction mechanism is the basis for treatments of most LSDs. The disease progression and improvement from treatment will be monitored in living animals by clinical neurological assessment, life-span increases, serum and CSF analyses, and non-invasive brain imaging by magnetic resonance spectroscopy (MRS) and diffusion tensor imaging (DTI). In post-mortem animals, the disease correction will be analyzed quantitatively for distribution of vector transduction, increases in MANB enzymatic activity, reduction of mannose-containing oligosaccharide substrate accumulation, histopathological changes, and potential adverse host responses. We will address important issues affecting translation by investigating strategies to attain more complete correction of the brain: in Aim 1 we will investigate the effects of alternative routes of CSF delivery; in Aim 2 we will investigate the effects of dose escalation on the extent of resolution of pathology; and in Aim 3 we will determine the effectiveness of therapy when initiated at progressively more severe stages of disease to evaluate the potential to ameliorate advanced disease.